badabing

I ran Lightning on Friday and the issue is in the braking zone before turn 1 and turn 6 (left hander)

Basically, holding the wheel straight and threshold brake from speed and the rear starts swaying side to side. Reduce pressure or just brake earlier/lighter and there is no issue.

rbahr

What are your suspension settings, has your LSD been checked recently?

mglobe

It may not be the issue, but before you jump too far down the rabbit hole, you may want to check your alignment. Could be a rear toe issue.

Bill Lehman

Over the years bumps have formed in the turn 1 brake zone. I had the same brake stability issue with my GT4 and believe it is rear bump steer. Adding rear toe helped the problem as does a good LSD. I don't notice the issue at 6 but am entering at a lower speed than 1.

badabing

I would to thank everyone for chiming in. To answer the questions and continue the discussion;

No LSD

Stock shocks/struts at oem non pasm ride height.

I have access to a current high end hunter alignment machine and I check/adjust alignment before every event.

The settings are adjusted with the car empty, half fuel, me sitting in the driver's seat.

Alignment is currently approx -2.0 camber all 4 corners.

Caster is approx 8.0

Front toe is 0 degrees. (I just changed it this past week from the stock .04). This is the outside limit of the factory tolerance.

I want to adjust the rear toe in as I've read in other threads on this issue but am not sure how much as the other threads talk about fractions of an inch, not degrees which is what this machine shows.

I also installed RSS rear toe arms to control bump steer but could not get a straight answer on how much to shim the outer links, if at all. Right now I've got about 12mm worth of shims in there.

This problem is track independent. Any really hard straight line braking does it.

WGI, Summit point, Tbolt, Lightning.

At T-1 Lightning I'm coming in about 135 mph. At T-6 about 100 or 105 mph.

My pace is 1:21.5 avg

Bill Lehman

My initial rear toe on the GT4 was -2.40 degrees. I later increased it. I tried to adjust bump on my 987 with shims. Ended up removing them. That car had a lowered ride height which may have made things worse in this regard.

jdistefa

You need a diff, period. That's going to be 90% of your issue.

Toe control arm shim stack for rear bump strikes me as too much given oem ride height. If you haven't already done so it's best to measure bump rather than guess.

Depending on the track you may want some front toe out. Forget about factory settings.

jdistefa

Totally disagree.

Are we talking race car on slicks or....?

badabing

If that's the case, why do you think this problem surfaced after replacing the MC

Im going to see if I can fashion up a bump steer gauge.


Do you have some specs you can recommend?

jdistefa

Likely because you can brake more in a shorter amount of time. Did you change the booster as well?

General principal is to check your alignment (including rake) first and then problem solve from there. But no LSD is going to result in an unstable rear end under braking. Period.


Measure your ride height first from wheel center to fender lip. Then put car on jackstands. Take the rear wheel off and then mount a laser pointer parallel to the floor on the rotor hat. Use a flat piece of cardboard to 'record' the beam. Start at full droop and then jack up by 1/2" and make a sharpie pen dot on the laser mark. Continue to full compression. Connect the dots and voila you have a reasonable bump curve. Note your original ride height so you can see where you currently sit on the curve.


It is totally track dependent. For something like the Glen with long straights it's fine to run 0 toe or maybe 0.5mm toe out each side. For something technical like NJMP or PittRace you could consider as much as 2mm toe out each side.

altonj

You can disagree. DE caymans/boxsters mostly, one race boxster S. R-compounds Nitto or Hoosier. What slicks are you running?

What pad set ups are you running?

I DO agree that a LSD (not TBD) will make a large difference. Some additional rear toe *may* make a difference.

How does a MC change allow you to brake more in a shorter amount of time?

Cheers

badabing

In my case I'm still on street tires



Booster was changed to PCCB model as well. This is a 987.2 CS w PDK. Unfortunately adding a LSD is not in the budget at the moment.

What do I do with the resulting bump curve?

This seems like front toe settings. Do you have a suggestion for rear toe settings considering the issue at hand?

recommendation?

jdistefa

1) The idea is to be on the flat[est] part of the bump curve in the functional range of droop and compression. This gives you the least dynamic toe change. This can be optimized by a combination of ride height and toe arm shims. Suggest you read Puhn's excellent book 'How to make your car handle'.

2) Rear toe-in beyond 3mm per side will help to somewhat stabilize the rear but will create more turn-in understeer. So you can end up chasing your tail to fix a problem that would be solved with an LSD (per altonj NOT a tbd).

jdistefa

Toyo RS1 per class rules.

Usually RS29 front with Brembo endurance rear. If a little more rear bite is desired for rotation then RS29 all around. Have used stock pads in the rear to mitigate ABS problems at low grip tracks like Mid-O.

Any more bite in the rear provokes ice mode. Having talked to many Cayman racers I don't know anyone in GTB, F, G, I, or SPC who runs more bite in the rear. Would certainly like to hear impressions if they do.

Totally understand that you may simply like more rear bite that works with your setup, tracks, preference (rotation).

MC and booster change allow later braking with more pedal pressure. Area under the curve is likely very similar to stock setup but time/velocity change is different hence more likely to provoke unsettling the rear end.

rbahr

Couple of thoughts:

The following will affect your braking:

• You went with a MC assembly designed for a car with more rear weigh bias
  
• You do not have a LSD: breaking loads will not be distributed in the rear
  
• Your suspension is likely a bit wonkey - you have no idea where your bump is currently at - AND you should do front and rear. What you are adjusting for is essentially no camber change over the region of interest

What was the original problem you were trying to solve?

Ray